Shading compensating mosaic screen electrode



April 19, 1949. s, ss e I 2,467,734

SHADING COMPENSATING MOSAIC SCREEN ELECTRODE Filed April 12, 1945 iilliilici INVENTOR SANFORD F. ESSIG N 1 TO PUMP E 5 Patented Apr. 19, 1949 SHADING COMPENSATING MOSAIC SCREEN ELECTRODE Sanford F. Essig, Fort Wayne, Ind., assignor, by

mesne assignments, to Farnsworth Research Corporation, a corporation of Indiana Application April 12, 1945, Serial No. 588,020

3 Claims.

1 This invention relates to a photosensitive cathode and a method of preparing the same, and more particularly relates to mosaic screen electrodes suitable for compensating shading in television picture signal generating tubes of the story age type.

It is well known in the art that in television picture signal generating tubes of the storage type spurious signals are developed which are commonly referred to as shading or black spot distortion. The spurious signals create improper background shading of the reproduced television image which must be compensated or suppressed. The shading effect may be considered to have two components. One component produces mainly a stationary black or dark shading of the reproduced image, while the other component creates shading distributed over the entire image area which depends upon the illumination of the tube.

It is not very well understood what causes these shading effects. However, various explanations have been suggested which at least qualitatively explain the occurrence of such spurious signals.

The mosaic screen electrode of a television picture signal generating tube of the storage type is,

scanned by a scanning beam in two directions according to the line frequency and the frame frequency. It sometimes happens that the scanning velocity of the scanning beam is not uniform due to imperfections of the deflecting system. It is believed that the non-uniform sweep velocity of the beam coupled with the direction of the scanning contributes to the stationary component of the shading. It is well known to those skilled in the art that the scanning beam of a conventional picture signal generating tube of the storage type impacts the mosaic screen electrode at an acute angle. This is also believed to be one of the causes of the shading effect. Due to the geometrical arrangement of a conventional television picture signal generating tube the electric fields between the mosaic screen and the other electrodes of the tube, particularly the collector or second anode, are not uniform. Another factor which contributes to an unequal field distribution across the surface of the mosaic screen is that the mosaic has a definite boundary, thus creating discontinuities of the electric potentials.

It has been observed experimentally that the shaded area of the reproduced image reverses its position when the direction of scanning of the scanning beam is reversed. This seems to indicate that the geometry of the picture signal generating tube and the direction of scanning of the beam are the main factors which cause this shading effect.

The unequal distribution of the electric field across the surface of the mosaic screen electrode causes a non-uniform redistribution of the secondary electrons liberated by the scanning beam impacting the mosaic. When these secondary electrons settle over certain areas of the mosaic, they drive these areas more negative than other areas which gives rise to spurious signals when the mosaic is scanned again by the electron beam.

Many suggestions have been made for compensating shading in a television picture signal generating tube of the storage type. It has, for instance, been proposed to compensate for the spurious signal by adding signals of variable phase, amplitude, frequency and wave shape to the video signal obtained from the tube. Such a system obviously requires a considerable amount of special equipment and a skilled operator for adjusting the compensating circuits. Furthermore, this arrangement does not prevent the occurrence of spurious signals at its source, but only suppresses them by adding suitable signals of opposite polari y.

It has also been suggested to utilize a mosaic screen electrode which will compensate for shading. To this end a mosaic is provided where the photosensitivity of the photosensitive particles of which the mosaic is composed varies in such a manner that shading is compensated. A mosaic of this type, however, has the drawback that the photcsensitivity of the materials customarily used for a mosaic has an optimum value, that is, where the photosensitivity exhibits a maximum. By varying the photosensitivity the amplitude of the video signal is decreased.

Furthermore, the photosensitivity and the secondary emission ratio are mutually dependent, and hence by varying the photosensitivity the secondary emission ratio will also be changed. As the liberation of secondary electrons from the mosaic screen by the scanning beam plays an important part in the generation of the video signal, changes of the secondary emission ratio may create undesired secondary effects resulting in a spurious signal.

The principal object of this invention is to provide in a television system a novel method and apparatus for compensating shading efiects.

It is a further object of the present invention to provide a novel mosaic screen electrode which will compensate for shading, and a method of preparing the same.

- In accordance with the present invention there is provided a mosaic screen electrode includin a metal support, a layer of dielectric material on the metal support and a photosensitive material on the dielectric layer. The dielectric layer varies in thickness according to a predetermined pattern. Hence, the capacitance between the metal support and the photosensitive material is varied for different unit areas.

In accordance'with the invention the method of preparing a mosaic screen electrode comprises depositing a continuous layer of dielectric material on a metal support in such a manner that the thickness of the dielectric layer varies in a predetermined manner. Then, a plurality of mutually separated photosensitive islands is formed upon the dielectric layer. In this manner the capacitance between the metal support and the photosensitive islands is varied for different unit areas by virtue of the varying thickness of the dielectric layer and shading is compensated.

For a better understandin of the invention, together with other and further objects thereof, reference is made to the following description, taken in connection with the accompanying drawing. and its scope will be pointed out in the appended claims.

In the accompanying drawing:

Fig. l is a plan view of a mosaic screen electrode in accordance with the present invention;

Fig. 2 is a cross-sectional view along line 2-2 of Fig. 1;

Fig. 3 is a cross-sectional View similar to Fi 2 of a modification of the mosaic of the invention; and

Fig. 4 illustrates apparatus for forming a mosaic electrode embodying the invention.

Referring now more particularly to Figs. 1 and 2 of the drawing, there is illustrated mosaic screen electrode l adapted particularly for a television picture signal generating tube and comprising metal support ll, dielectric layer 12 and photosensitive particles [3. Metal support ll serves for developing the video signal in a conventional manner and is commonly referred to as the signal plate. Photosensitive particles l3 are mutually separated from each other and may be prepared in a conventional manner such as by oxidizing silver droplets and thereafter sensitizin them with caesium. Dielectric material 52 may consist of any suitable insulating material including certain fluorides such as barium fluoride and calcium fluoride having the formula BaFz and CaFc, respectively. Another suitable dielectric material is quartz.

In accordance with the present invention the thickness of dielectric layer I2 is varied. Thus, the capacitance between signal plate H and photosensitive particles I3 may be varied for different unit areas to compensate shading. The area above and to the left of diagonal I4 is shaded in. Fig. 1 to indicate the shaded area in the reproduced television image. The shaded area, of course, does not have a well defined boundary line. Arrow I 5 indicates the horizontal scanning direction, and arrow [6 indicates the vertical scanning direction of the scanning beam. As stated hereinabove, it has been found experimentally that the shaded area in the reproduced picture occurs at the opposite side of diagonal [4 when the scanning direction is reversed.

Background shading of the reproduced television picture is caused by spurious signals which are superimposed upon the charge image representative of the object to be televised. The useful video signal is developed as a result of the potential difference of an illuminated area and a dark area of the mosaic which are brought to the same reference potential by the scanning beam. Experiments have been carried out with a television picture signal generating tube including a mosaic screen electrode with a dielectric layer, such as It. in Fig. 2, having a thicker portion and a thinner portion. It has been found that a television image reproduced by means of the teledefinitely darker area where the dielectric layer of the mosaic screen electrode is thinner. Hence,

in accordance with the invention dielectric layer I2 is made thicker where shading normally 5 occurs to compensate for the shading effect. As shown in Fig. 2, the thickness of dielectric layer l2 may increase gradually and linearly from the right towards the left. A cross-section taken along the vertical center line of mosaic 10 would look substantially the same as the cross-section illustrated in Fig. 2.

Fig. 3 shows a cross-section of a modified mosaic screen electrode. Mosaic ll includes signal plate I8, dielectric layer 29 and photosensitive islands 25. The thickness of dielectric layer 20 increases non-linearly from the right towards the left of Fig. 3. Thus, the capacitance between signal plate I8 and photosensitive particles 2| or between signal plate II and photosensitive is- 20 lands 13 may be varied for different unit areas in a manner to compensate for shading. The capacitance per unit area of the mosaic screen of a television picture signal generating tube of the storage type is not critical and may be varied between the limits of one to six without impairing the operation thereof. As the capacitance of the mosaic screen is inversely proportional to the thickness of its dielectric layer for a given material, variation of the thickness thereof will cause a proportional change of the capacitance per unit area.

Dielectric layer E2 or of variable thickness may be prepared in different ways. One method of preparing such a dielectric layer is illustrated in Fig. 4. As shown in Fig. 4, there is provided an envelope having a tubular extension 26 which may be connected to a vacuum pump for exhausting the same. Metal support 21 is mounted in envelope 25 at an acute angle with respect to dielectric bead 28 heated by tungsten wire 39 which may be connected to a source of potential, not shown. At first, envelope 25 is exhausted through tubular extension 26, whereafter heater wire 30 is heated until dielectric head 28 evaporates.

As metal support 2! is arranged at an acute angle with respect to evaporator '28, the dielectric material is condensed on metal support 21 with uneven thickness. Equal amounts of the dielectric material evaporated from bead 28 are deposited on the areas covered by equal solid angles, such as indicated at 3| and 32. Thus, it will be seen that the condensed dielectric layer will be much thinner at the right hand portion of metal support 21, which is at a greater distance from evaporator 28, than at the left hand portion thereof. It is also feasible to provide more than one evaporator in order to obtain any desired distribution of the dielectric material on metal support 21.

Barium fluoride and calcium fluoride are suitable dielectric materials which may be evaporated in this manner. Alternatively, quartz or any other dielectric material which evaporates 65' Without decomposing may be used as the dielectric of the mosaic screen. After the dielectric layer is formed on metal support 21, photosensitive particles are formed on the dielectric layer in a conventional manner.

It is also feasible to interpose stationary or rotating masks between evaporator 28 and metal support 21 to obtain a predetermined thickness distribution of the dielectric layer. Such masks have been disclosed, for example, in the Patents vision signals obtained from this tube shows a 2175 .No; 2,341,827 of February 15, 1944 and No. 2.384;-

209 of September 4, 1945 both to Sukumlyn. Thus, any desired values of the capacitance per unit area between signal plate II and photosensitive islands |3 or signal plate I8 and photosensitive particles 2| may be obtained. Preferably, the thickness of dielectric layer l2 decreases gradually from the two edges 33 and 34 of Fig. 1 towards diagonal [4.

While there has been described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A mosaic screen electrode including a metal support, a dielectric layer on said metal support, and a plurality of mutually separated photosensitive particles on said dielectric layer electrically insulated from said metal support, said dielectric layer varying in thickness gradually and linearly from two adjacent edges of said support towards the other two edges thereof so that the capacitance between said metal layer and said photosensitive particles varies for different unit areas, thereby to compensate for shading.

2. A mosaic screen electrode including a metal support, a dielectric layer on said metal support, and a plurality of individual photosensitive islands on said dielectric layer electrically insulated from said metal support, said dielectric layer varying in thickness gradually and non-linearly from two adjacent edges of said support towards the other two edges thereof so that the capacitance between said metal layer and said photosensitive islands varies for different unit areas, thereby to compensate for shading.

3. A mosaic screen electrode including a metal support of substantially rectangular shape, a layer of dielectric material on said metal support, and a plurality of mutually separated photosensitive particles on said dielectric layer electrically insulated from said metal support, said dielectric layer increasing gradually in thickness from two adjacent edges of said support towards the other two edges thereof so that the capacitance between said metal layer and said photosensitive particles varies in a predetermined manner for different unit area-s, thereby to compensate for shading.

SANFORD F. ESSIG.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,969,558 Ives Aug. 7, 1934 2,121,356 Knoll June 21, 1938 2,175,691 Iams Oct. 10, 1939 2,238,381 Batchelor Apr. 15, 1941 2,251,992 Flory et a1. Aug. 12, 1941 2,282,123 Essig May 5, 1942 2,285,058 Samson June 2, 1942 2,341,827 Sukumlyn Feb. 15, 1944 2,384,209 Sukumlyn Sept. 4, 1945 

